The present invention relates to a sintered hard metal and more particularly to cemented carbide having a superior toughness which may be utilized for micro-drills for drilling printed circuit boards, cutting tools, mining tolls and wear-resistant parts.
Tools made of cemented carbide, including WC-Co (tungsten carbide-cobalt) as a typical composition, are widely used in the machining field. The alloy compositions, characteristics, uses and applications of such cemented carbide materials are summarized in the "Cemented Carbide Tool Handbook" published by the Japan Cemented Carbide Tool Manufacturers Association on Sept. 10, 1976.
In WC-Co cemented carbide, a higher concentration of Co as a metal phase provides a higher transverse strength to increase the "toughness" transverse rupture strength, but lowers the hardness, i.e., decreases wear resistance. In order to improve such defects, TiC (titanium carbide) TaC (tantalum carbide), NBC, etc., are contained as a hard phase. As to cemented carbide commercially available on the market, however, the highest transverse rupture strength is about 300 to 320 kgs/mm.sup.2 and the highest hardness HRA is about 80 to 85 (above-referenced Handbook, page 11, Table 1.6). That is, as the transverse strength is increased, the hardness is lowered; or as the hardness is increased, the transverse strength is lowered.
As to cemented carbide having a high hardness and a high toughness, the U.S. Pat. No. 3,480,410 discloses a WC-CrC-Co sintered composite having a toughness higher than that of normal WC-base cemented carbide and having a hardness higher than that of normal cemented carbide containing a large amount of Co. In the cemented carbide of the '410 patent, chromium carbide is present in the amount of 0.1 to 2.5 percent by weight, in the form of a complete dispersion of extremely fine grain size in the range of 0.2 micron, and the cobalt is present in a range of 9 to 20 percent by weight. The specification of the '410 patent describes that the alloy of its Example 1 has a Rockwell Hardness HRA of 91.7 and a transverse rupture strength of 315 kgs/mm.sup.2 (unit conversion), and the alloy made in Example 2 has a Rockwell Hardness HRA of 90.5 and a transverse rupture strength of 353 kgs/mm2.
Micro-drills capable of drilling deep holes with a small diameter are increasingly needed for the precise drilling of printed electronic circuit board. There is a trend to use cemented carbide for such drills. An alloy having a higher hardness and higher toughness than that of conventional cemented carbide is therefore highly desirable. Micro-drills have a relatively long length compared to their diameter. Small-diameter drills, of 0.05 to 0.5 mm.phi., often cause fracture accidents when used in high-speed drilling. In order to enhance the rate of operation of machines, increased wear resistance of drills, and other parts subject to friction, is desirable.